Refactor existing work item widget services to callback classes

This is a follow-up from #368503 (closed).

Looking at app/services/work_items/widgets/, we have:

• Assignees (!140001 (merged))
• Award emoji (!121366 (merged))
• Current user todos (!141186 (merged))
• Description (!140577 (merged))
• Hierarchy (!162409 (merged))
• Labels (!162520 (merged))
• Notifications (!140004 (merged))
• Start and due date (!141727 (merged))

In ee/app/services/work_items/widgets/, we have:

• Color !142335 (merged)
• Health status !149536 (merged)
• Iteration !160038 (merged)
• Notes !142290 (merged)
• Progress !160057 (merged)
• Status !161266 (merged)
• Weight !161267 (merged)
• Rolledup Dates !161269 (merged)